Golf club

The present invention relates to a golf club which can control a decrease in the rebound performance on off-center hit by defining a thickness distribution of a face portion in consideration of a toe down during swing, more particularly to a golf club which can be suitably used by average golfers whose head speed is not more than 40 m/s.

In recent years, there has been proposed a golf club head having a hollow structure comprising a face portion provided with a central thick part and a thin part surrounding the central thick part for example as disclosed in Japanese patent application publications Nos. JP-2010-104473-A and JP-2001-579915-A. Owing to the thin part, such a golf club has an advantage such that a decrease in the rebound of the ball on off-center hit can be controlled.

By the way, as shown in FIG. 11 , due to the structure of the golf club (a), the center G of gravity of the club head (b) is positioned at a certain distance from the center line (d) of the shaft (c). Accordingly, during swing, due to the centrifugal force, the club head (b) moves closer to the swing plane. As a result, the shaft (c) is bent, and the toe b 1 of the club head (b) comes down (toward the ground) when compared with the position at address. Thus, so called toe-down phenomenon is caused. With increase in the toe-down, the golf ball hitting positions vary wide in the toe-heel direction of the face. The present inventor, therefore, studied on the toe-down during swing quantitatively in relation to the reverse flex of the club and discovered that the decrease in the rebound performance on off-center hit can be minimized by specifically defining the thickness distribution of the face portion based on the reverse flex.

The present invention was studied out with the view to the current conditions stated above and mainly intended to provide a golf club which control the decrease in the rebound performance on off-center hit by defining a distribution of thin parts suitably for variations of ball-hitting-positions based on the reverse flex.

According to the present invention, a golf club comprises:

a shaft; and

a golf club head with a hollow structure fixed to one end side of the shaft and including a face portion having a face for hitting a ball,

wherein

a reverse flex is 110 to 160 mm,

the face portion includes a central thick part provided in a central region, a toe-crown-side thin part of a small thickness provided on a crown-side and on a toe-side of the central thick part, and a heel-sole-side thin part of a small thickness provided on a sole-side and on a heel-side of the central thick part, and

in a front view in a standard state put on a horizontal plane at a specified lie angle and loft angle, a first straight line passing an area centroid of the toe-crown-side thin part and an area centroid of a back surface of the face portion has an angle θA of 10 to 30 degrees with respect to the horizontal plane, and a second straight line passing an area centroid of the heel-sole-side thin part and the area centroid of the back surface of the face portion has an angle θB of 36 to 40 degrees with respect to the horizontal plane.

Further, it is also possible that the toe-crown-side thin part and the heel-sole-side thin part have a thickness of 1.8 to 2.4 mm, the area of the toe-crown-side thin part is 8 to 20% of the overall area of the back surface of the face portion, and the area of the heel-sole-side thin part is 3 to 10% of the overall area of the back surface of the face portion, that the volume of the golf club head is 400 to 470 cc. and that the golf club head is of a wood-type.

The golf club according to the present invention is a golf club comprising the shaft and the golf club head with the hollow structure fixed to one end side of the shaft and including the face portion having the face for hitting a ball, and the reverse flex is limited to 110 to 160 mm. In such golf club, when swung at a head speed of not more than 40 m/s, more specifically 34 to 40 m/s for example, the amount of toe down falls within a substantially fixed range. Therefore, based on this, it is possible to estimate the range of variations of ball-hitting-positions in the toe-heel direction of the face portion. Therefore, according to the present invention, in the front view of the club in the standard state put on the horizontal plane at the specified lie angle and loft angle, the angle θA of the first straight line passing the area centroid of the toe-crown-side thin part of the face portion and the area centroid of the back surface of the face portion is limited to 10 to 30 degrees with respect to the horizontal plane, and the angle θB of the second straight line passing the area centroid of the heel-sole-side thin part and the area centroid of the back surface of the face portion is limited to 36 to 40 degrees with respect to the horizontal plane. In such golf club head, the thin parts of the face portion whose rebound characteristic is high are arranged in a specific distribution suitable for the variations of the ball-hitting-positions caused by the toe down corresponding to the amount of the toe down, therefore, the decrease in the rebound performance on off-center hit can be minimized.

FIG. 1 is a front view of a golf club according to an embodiment of the present invention in its standard state.

FIG. 2 is a partial plan view of the golf club shown in FIG. 1 .

FIG. 3 is an exploded perspective view of the golf club head before assembled.

FIG. 4 is a diagram for explaining a method for measuring the reverse flex.

FIGS. 5( a )- 5 ( c ) are graphs showing correlations of the reverse flex and the carry distance and directionality at each head speed.

FIG. 6 is a front view of FIG. 2 .

FIG. 7 is a back view of a face member showing the back surface of the face portion.

FIG. 8 is an enlarged A-A sectional view of FIG. 6 .

FIG. 9 shows an example of distributions of ball-hitting-positions of average golfers.

FIG. 10 is a front view of a golf club according to another embodiment of the present invention.

FIG. 11 is a front view for explaining the toe down.

An Embodiment of the present invention will now be described in detail in conjunction with accompanying drawings.

In the drawings, a golf club 1 according to the present invention comprises a shaft 2 , a golf club head 3 attached to one end side 2 A of the shaft 2 , and a grip 4 attached to the other end side 2 B of the shaft 2 .

In this embodiment, as shown in FIG. 1 and FIG. 2 , the golf club head 3 is formed as a wood-type golf club head such as for driver (# 1 ), spoon(# 3 ) and the like.

In FIG. 1 and FIG. 2 , shown is the golf club 1 in its standard state. The standard state is such that the golf club head is placed on a horizontal plane HP so that the center line CL of the golf shaft 2 is inclined at its lie angle alpha while keeping the center line CL on a vertical plane VP, and the face 5 (at the sweet spot SS) forms its loft angle with respect to the horizontal plane HP. In the description, the golf club is described based on that it is being in the standard state unless otherwise noted. The loft angle is given as being more than 0 degree. The sweet spot SS is the point of intersection between the face 5 and a straight line n drawn normally to the face from the center of gravity G of the golf club head.

The club length L of the golf club 1 in this embodiment is not limited. But, if the club length L of the golf club 1 becomes excessively increased, the swing balance becomes worse, and variations of ball-hitting-positions increase. If the club length L becomes excessively decreased it becomes difficult to increase the golf club head speed by utilizing the club length. In this light, the club length L is preferably not less than 45 inches, more preferably not less than 45.5 inches, but not more than 47 inches, more preferably not more than 46.5 inches.

The club length L of the golf club 1 is, as shown in FIG. 1 , a length L measures along the center line CL of the shaft 2 from the grip side end 2 e of the shaft 2 to the intersecting point X of the center line CL of the shaft 2 with the horizontal plane HP in the standard state.

It is preferable that the shaft 2 is made of a fiber reinforced resin material. Such shaft 2 is lightweight and thereby it is easy to swing through the golf ball, and it has a high flexibility in designing such as adjusting of the weight balance and the amount of deflection and the like, therefore, it is desirable. However, a metal material may be used for the shaft 2 .

As shown in FIGS. 2 and 3 , the club head 3 comprises a face portion 6 of which front surface defines a face 5 for striking the golf ball, a crown portion 7 defining the top surface of the club head intersecting the face 5 , a sole portion 8 defining the bottom face of the club head intersecting the face 5 , a side portion 9 between the crown portion 7 and sole portion 8 extending from the toe-side edge 5 c of the face 5 to the heel-side edge 5 b of the face 5 passing through the back face BF of the golf club head, and a tubular hosel portion 10 having a shaft inserting hole 10 e positioned on the heel-side of the crown portion 7 . And the club head 3 is constructed as a hollow structure provided therein with a hollow (i).

The club head 3 may have a two-piece structure, three-piece structure, four-piece structure and the like. In this embodiment, as shown in FIG. 3 , the club head 3 has a two-piece structure composed of a main body member 3 B and a face member 3 A attached to the front of the main body member 3 B and forming the face portion 6 .

The face member 3 A and the main body member 3 B can be made from various metal materials. For example, one or two or more kinds of titanium alloys, stainless steel or steel alloys, and the like can be suitably used. In combination with such metal material(s), a light weight material having a specific gravity lower than the metal material(s) such as fiber reinforced resin can be used to form a part of the club head 3 .

The face member 3 A is for example formed in a substantially cup-like shape including the entirety of the face portion 6 and an extension portion 11 extending backwardly of the club head from each edge 5 a - 5 d of the face 5 for a small length. The extension portion 11 includes a crown-side extension portion 11 a , a sole-side extension portion 11 b , a toe-side extension portion 11 c and a heel-side extension portion 11 d . In the face member 3 A in this embodiment, all portions are integrally formed by pressing a rolled material to cause a plastic deformation, for example.

The main body member 3 B includes the part of the club head 3 excluding the face member 3 A. More specifically, the main body member 3 B integrally includes a crown aft part 7 b , sole aft part 8 b and side aft part 9 b which correspond to major aft parts of the crown portion 7 , sole portion 8 and side portion 9 , respectively. Further, the hosel portion 10 is also included. The main body member 3 B is integrally molded by casting for example.

If the volume V of the club head 3 is excessively decreased, a sweet spot area becomes decreased, and the rebound of a ball on off-center hit is liable to decrease. If the volume V is excessively increased, as the mass of the club head increases, it becomes difficult swing through the ball and the golf club head speed decreases. In this light, the volume V is preferably set in a range of not less than 400 cc, more preferably not less than 410 cc, but not more than 470 cc, more preferably not more than 460 cc.

If the mass of the club head 3 is excessively decreased, the kinetic energy of the club head decreases, and it becomes difficult to improve the carry distance. If the mass of the club head 3 is excessively increased, it becomes difficult swing through the ball and the carry distance tends to decrease. In this light, the mass of the club head 3 is preferably set in a range of not less than 180 g, more preferably not less than 185 g, but not more than 210 g, more preferably not more than 200 g.

The grip 4 is formed from a vulcanized rubber made from natural rubber, oil, carbon black, sulfur, zinc oxide and the like for example. It is preferable that such grip 4 has a mass of 38 to 46 g.

According to the present invention, the golf club is designed to minimize the decrease in the carry distance on off-center hit when the golf club head speed is 34 to 40 m/s, and thus it can be suitably used by nonpowerful average golfers such as aged golfers and lady golfers for example. Based on this standpoint, the reverse flex Ry of the golf club 1 is set in a range of from 110 to 160 mm.

The reverse flex Ry is, as shown in FIG. 4 , the amount of deflection of the club measured at a point P 1 on the grip side as a displacement in the vertical direction when the club 1 is supported at points S 1 and S 2 on the club head side so that the center line CL of the shaft 2 becomes parallel with the horizontal direction and a load W 1 of 1.25 kgf is applied downwardly to the above-mentioned point P 1 , wherein

the point S 1 is positioned at 40 mm from the above-mentioned intersecting point X (shown in FIG. 1 ), the point S 2 is positioned at 140 mm from the point S 1 , the point P 1 is positioned at a distance Ld from the point S 2 , and the distance Ld is as follows:

driver (# 1 ): 860 mm brassie (# 2 ): 847 mm spoon (# 3 ): 835 mm baffy (# 4 ): 822 mm cleek (# 5 ): 809 mm

(# 7 ): 796 mm

FIGS. 5( a )- 5 ( c ) show results of actual ball hitting tests carried out by ten golfers whose average head speed was 34 m/s, ten golfers whose average head speed was 40 m/s and ten golfers whose average head speed was 42 m/s. In the tests, the reverse flex Ry was changed, but other specifications were not changed. (club length 47 inches, club mass 300 g , loft angle 11 degrees, head volume 455 cc, thick part thickness 3.4 mm, thin part thickness 2.0 mm) As apparent from the test results, when the reverse flex Ry is less than 110 mm, the shaft 2 becomes hard for the golfers with slow swing, and in the above-mentioned head speed range, it is difficult to appropriately bend the shaft to obtain a sufficient carry distance, and further, it becomes difficult for the face 5 to return its direction to that at the address. Thus, the directional stability of the struck balls becomes worse. If the reverse flex exceeds 160 mm, as the shaft 2 becomes soft, it becomes difficult for the above-mentioned nonpowerful golfers to stably control the direction of the face 5 , therefore, there is a tendency that the directional stability of the struck balls is remarkably deteriorated. In this light, the reverse flex is more preferably not less than 120 mm, but not more than 150 mm. As explained above, in the present invention, the reverse flex Ry is optimized based on the golf club head speed range of nonpowerful golfers, to obtain a sufficient carry distance and directional stability.

Such reverse flex Ry can be easily adjusted within the above range by changing the kind of the material of the shaft 2 , the elastic modulus of the material of the shaft 2 and the like.

The present inventor discovered through experiments that, by specifically arranging the thickness distribution of the face portion 6 in relation to the reverse flex Ry in the above range, it is possible to avoid the decrease in the carry distance on off-center hit. More specifically, when the golfers whose average head speed ranges from 34 to 40 m/s use golf clubs having a reverse flex of from 110 to 160 mm, the amount of toe down falls in a substantially fixed range, and the range of variations of ball-hitting-positions on the face is also substantially fixed, therefore, the decrease in the carry distance (rebound performance) on off-center hit can be minimized by increasing the restitution coefficient locally in such variations' range of the face portion 6 .

Next, such thickness distribution capable of increasing the rebound performance will be described concretely.

In FIG. 6 , the front view of the face member 3 A under the standard state is shown. FIG. 7 shows the back surface 6 B of the face member 3 A. FIG. 8 shows the cross section of the face member 3 A taken along line A-A in FIG. 6 . As shown, the face 5 is smooth except for face grooves, punch marks and the like (not shown). In contrast, the back surface 6 B of the face portion 6 is nonsmooth so that the face portion 6 has a specific thickness distribution.

In this embodiment, the face portion 6 comprises a central thick part 15 including the sweet spot SS, a toe-crown-side thin part 16 on the toe-side and on the crown-side of the central thick part 15 , a heel-sole-side thin part 17 on the heel-side and on the sole-side of the central thick part 15 , a heel-side middle thickness part 18 on the heel-side of the central thick part 15 , and a toe-side middle thickness part 19 on the toe-side of the central thick part 15 .

The central region of the face portion 6 is subjected to a large impulsive force when hitting a ball. Therefore, in order to improve the durability of the face portion 6 , the central thick part 15 has a largest thickness in the face portion 6 . Here, the central region of the face portion 6 is a region having a certain area including the area centroid SG of the contour shape at the peripheral edge 5 e of the back surface of the face portion 6 facing the hollow (i). The peripheral edge 5 e of the back surface of the face portion is, as shown in FIGS. 7 and 8 , given as a boundary line between the back surface 6 B of the face portion 6 and the inner surfaces of the crown portion 7 , sole portion 8 and side portion 9 , and in this embodiment, it is constructed to include a crown-side peripheral edge 5 e 1 of the back surface of the face extending on the crown-side and a sole-side peripheral edge 5 es of the back surface of the face extending on a sole-side. If the back surface 6 B is connected to the inner surfaces of the portions 7 - 9 through an arc like a chamfer to prevent stress concentration, then for the purpose of convenience, the peripheral edge 5 e of the back surface of the face is defined as the midpoint of the length of the arc R in a cross section of the club head.

In this embodiment, the central thick part 15 is composed of a base 15 a which is generally a horizontally long oval along the contour shape of the peripheral edge 5 e of the back surface of the face portion, an upward rib 15 b extending from an upper part of the base 15 a on its heel-side to the crown-side peripheral edge 5 e 1 of the back surface of the face, while inclining to the heel-side, and a downward rib 15 c extending from a lower part of the base 15 a on its toe-side to the sole-side peripheral edge 5 e 2 of the back surface of the face, while inclining to the toe-side. Owing to the base 15 a , such central thick part 15 has an advantage capable of securing the durability of the central portion region which is most deflectable in the face 5 . Further, since both of the ribs 15 b and 15 c are formed parallel with the center line CL of the shaft, they can balance the moment of inertia around the center line CL and make it easy to swing.

In order to certainly improve the durability of the face portion 6 , the thickness tc of the central thick part 15 is set to be preferably not less than 3.1 mm, more preferably not less than 3.2 mm. On the other hand, if the thickness tc of the central thick part 15 becomes excessively increased, there is a possibility of deterioration in the rebound performance and that, due to the increased face weight, the swing balance is disturbed and ball-hitting-positions vary wide. In this light, the thickness tc of the central thick part 15 is preferably not more than 3.7 mm, more preferably not more than 3.6 mm. It is preferable that the thickness of the central thick part 15 is substantially constant.

In this embodiment, the thickness of the upward rib 15 b and the thickness of the downward rib 15 c are substantially the same as the thickness of the base 15 a . However, it may be possible that, in order to reduce the mass of the club head 3 , the thickness of the upward rib 15 b is gradually decreased toward the crown-side, and the thickness of the downward rib 15 c is gradually decreased toward the sole-side.

In order to secure the durability of the face portion 6 and suppress the deterioration in the rebound performance and the increase in the weight of the face portion, the area MC of the central thick part 15 is preferably set in a range of not less than 10%, more preferably not less than 15%, but not more than 30%, more preferably not more than 25% of the overall area MG of the back surface 6 B of the face portion 6 . For the sake of convenience, the areas of the back surface 6 B of the face portion 6 and the central thick part 15 (as well as the areas of the after-mentioned parts 16 - 19 ) are each defined by an area obtained by a two-dimensional shape of such region or part projected on the vertical plane VP shown in FIG. 2 (or a vertical plane parallel thereto).

The overall area MG of the back surface 6 B is set based on the volume of the club head 3 but preferably set in a range of not less than 33 sq.cm, more preferably not less than 35 sq.cm, but not more than 53 sq.cm, more preferably not more than 47 sq.cm.

The toe-crown-side thin part 16 and the heel-sole-side thin part 17 are formed to have a constant thickness in this embodiment, and have the smallest thickness in the face portion 6 . Therefore, even if the ball hitting position is in a toe-side or a heel-side in the club face, namely, on off-center hit, the face portion 6 makes a large elastic deformation, and the decrease in the rebound performance and the decrease in the carry distance can be avoided.

Through experiments conducted by the inventor, it was discovered that, when golfers whose head speed ranges from 34 to 40 m/s use golf clubs having a reverse flex Ry of from 110 to 160 mm, the amount of toe down falls within a substantially fixed range. FIG. 9 shows ball hitting positions of such golfers. As shown, the ball hitting positions of the golfers whose head speed is relatively low have a tendency to vary from the area centroid SG of the back surface 6 B toward the toe-side at a certain angle and toward the heel-side at a certain angle. In the golf club 1 according to the present invention, therefore, the toe-crown-side thin part 16 and the heel-sole-side thin part 17 are formed to accord with this distribution in order to minimize the decrease in the rebound performance due to the variations of the ball-hitting-positions.

Concretely speaking, as shown in FIG. 6 , in the front view under the standard state, a first straight line K 1 passing the area centroid SA of the toe-crown-side thin part 16 and the area centroid SG of the back surface 6 B of the face portion 6 has to have an angle θA of from 10 to 30 degrees with respect to the horizontal plane HP, and a second straight line K 2 passing the area centroid SG and the area centroid SB of the heel-sole-side thin part 17 has to have an angle θB of from 36 to 40 degrees with respect to the horizontal plane HP. Thereby, it becomes possible to arrange the toe-crown-side thin part 16 and the heel-sole-side thin part 17 in suitable positions corresponding to the variations of ball hitting positions according to the amount of toe down of the golf club 1 occurring due to the reverse flex. The above-mentioned front view is specified as the shape of the club head 3 when the face 5 is viewed form a direction orthogonal to the vertical plane VP as shown in FIG. 2 by symbol F. The area centroids SA, SB and SG are obtained based on the after-mentioned areas MA, MB and MG.

If the angle θA is less than 10 degrees or more than 30 degrees or the angle θB is less than 36 degrees or more than 40 degrees, then the positions of the thin parts 16 and 17 do not match with the ball-hitting-positions corresponding to the amount of toe down, and the rebound performance on off-center hit is decreased. In this light, the angle θA is preferably not less than 15 degrees, but not more than 25 degrees, and the angle θB is preferably not less than 37 degrees, but not more than 39 degrees.

If the thickness ta of the toe-crown-side thin part 16 and the thickness tb of the heel-sole-side thin part 17 are excessively decreased, there is a possibility that the durability of the face 5 is deteriorated. If excessively increased, there is a possibility that the rebound performance is deteriorated. Therefore, the thicknesses ta and tb of the thin parts 16 and 17 are preferably set in a range of not less than 1.8 mm, more preferably not less than 1.9 mm, but not more than 2.4 mm, more preferably not more than 2.2 mm. If the face 5 is provided with face grooves and/or punch marks, the thickness of each part of the face portion 6 is measured in such a condition that the face grooves and punch marks are filled.

If the area MA of the toe-crown-side thin part 16 and the area MB of the heel-sole-side thin part 17 become decreased, there is a possibility that the improvement in the rebound performance of the club head becomes insufficient. If the area MA and area MB become increased, there is a possibility that the durability of the club head 3 is decreased. In this light, the area MA of the toe-crown-side thin part 16 is preferably set in a range of not less than 8%, more preferably not less than 10%, but not more than 20%, more preferably not more than 18% of the overall area MG of the back surface 6 B. And the area MB of the heel-sole-side thin part 17 is preferably set in a range of not less than 3%, more preferably not less than 5%, but not more than 10%, more preferably not more than 8% of the overall area MG of the back surface 6 B. Especially, it is preferable that the area MA of the toe-crown-side thin part 16 is more than the area MB of the heel-sole-side thin part 17 .

In this embodiment, the toe-crown-side thin part 16 is formed in a region between two straight lines intersecting at the area centroid SG at an angle alpha A. This angle alpha A is preferably set in a range of not less than 60 degrees, more preferably not less than 75 degrees, but not more than 120 degrees, more preferably not more than 100 degrees. Similarly, the heel-sole-side thin part 17 is formed in a region between two straight lines intersecting at the area centroid SG at an angle alpha B. This angle alpha B is preferably set in a range of not less than 60 degrees, more preferably not less than 70 degrees, but not more than 120 degrees, more preferably not more than 100 degrees. Thereby, the rebound performance and the durability of the face 5 can be improved in a well balanced manner.

The middle thickness parts 18 and 19 can prevent the occurrence of large rigidity difference between the central thick part 15 and the thin parts 16 and 17 , and a stress concentration thereon can be effectively prevented. Thereby, it is possible to surely avoid the deterioration in the durability of the face portion 6 .

In order to achieve the durability of the face portion 6 and the prevention of weight increase of the club head 3 in a well balanced manner, the total area MS of the middle thickness parts 18 and 19 is preferably set in a range of not less than 10%, more preferably not less than 20%, but not more than 60%, more preferably not more than 50% of the overall area MG of the back surface 6 B of the face portion 6 .

From the same standpoint, the thickness of each of the middle thickness parts 18 and 19 is preferably set in a range of not less than 45%, more preferably not less than 50%, but not more than 85%, more preferably not more than 80% of the thickness tc of the central thick part 15 .

On the toe-side of the central thick part 15 , there is provided with a toe-side central thickness transitional part 20 a which extends semicircularly and of which thickness continuously decreases toward the peripheral edge 5 e of the back surface of the face portion. On the heel-side of the central thick part 15 , there is provided with a heel-side central thickness transitional part 20 b which extends semicircularly and of which thickness continuously decreases toward the peripheral edge 5 e of the back surface of the face portion.

Further, between the upward rib 15 b and the heel-side middle thickness part 18 , between the upward rib 15 b and the toe-crown-side thin part 16 , between the downward rib 15 c and the toe-side middle thickness part 19 and between the downward rib 15 c and the heel-sole-side thin part 17 , there is provided with an outside thickness transitional part 21 a of which thickness continuously decreases from the central thick part 15 . Further, between the heel-side middle thickness part 18 and the heel-sole-side thin part 17 , and between the toe-crown-side thin part 16 and the toe-side middle thickness part 19 , there is provided with a small outside thickness transitional part 21 b narrower in width than the outside thickness transitional part 21 a . These thickness transitional parts can prevent the occurrence of large rigidity difference due to the thickness difference and stress concentration is prevented, which helps to improve the durability of the face portion 6 . In this embodiment, each thickness transitional part is formed to have a substantially constant width.

A detailed description of the present invention is given as above. The present invention is not to be limited to the above-described specific embodiment. Rather, it can be modified variously if desired. For example, as shown in FIG. 10 , it is possible to dispose between the upper rib 15 b and the toe-crown-side thin part 16 , a toe-crown-side middle thickness part 22 extending along the peripheral edge 5 e of the back surface of the face portion.

[Embodiments]

In order to confirm the effects of the present invention, carbon shafts (SP600, flex R) manufactured by SRI Sports Limited were attached to wood-type golf club heads (driver) based on the specifications shown in FIG. 6 and Table 1, and wood-type clubs having club lengths of 45 to 47 inches were experimentally produced and tested for the rebound performance. Each of the club heads had a two-piece structure formed by laser welding a main body member as a lost-wax precision casting of Ti-6Al-4V and a face member of a cup-like shape as a press molded product of Ti-6Al-4V. They had the same parameters except for the parameters shown in Table 1, and major common specifications are as follows. The angles θA and θB were changed while maintaining a constant club mass.

lie angle alpha: 58 degrees loft angle beta: 10.5 degrees head volume V: 455 cc club head mass: 190 g overall area MG of back surface of face portion: 46.4 sq.cm thickness tc of central thick part: 3.35 mm area ratio MC/MG of central thick part and back surface of face portion: 18 to 22% thickness to of toe-crown-side thin part: 2.0 mm area ratio MA/MG of toe-crown-side thin part and back surface of face portion: 10 to 16% thickness tb of heel-sole-side thin part: 2.0 mm area ratio MB/MG of heel-sole-side thin part and back surface of face portion: 5 to 10% thicknesses of heel-side middle thickness part and toe middle thickness part/thickness of central thick part: 50 to 70% area ratio MS/MG of total area of middle thickness parts and back surface of face portion: 15 to 25%

In each of the thickness transitional parts, the thickness was smoothly changed.

The test method was as follows.

<Rebound Performance>

With respect to each of the above-mentioned test clubs, ten testers (average golfers) (head speed 34 to 40 m/s) each hit ten balls by the use of a golf club having a length selected according to own choice, and the golf club head speed HS immediately before hitting a ball and the initial speed BS of the ball were measured to calculate the average of the speed ratios BS/HS. As to the golf balls, three-piece golf balls commercially available as “XXIO” manufactured by SRI Sports Limited were used. The larger the value, the better the results. The results of the test and other are shown in Table 1. In Table 1, the “toe portion gravity center angle θA” means the angle of the first straight line K 1 passing the area centroid SA of the toe-crown-side thin part 16 and the area centroid SG of the back surface 6 B of the face portion 6 with respect to the horizontal plane HP. And the “heel portion gravity center angle θB” means the angle of the second straight line K 2 passing the area centroid SB of the heel-sole-side thin part 17 and the area centroid SG of the back surface 6 B of the face portion 6 with respect to the horizontal plane HP.

The average head speed of the testers and the club length used are shown in Table 2.

From the test results, it can be confirmed that the golf clubs as Embodiments were significantly improved in the rebound performance in comparison with the comparative examples. Further, changing the area ratio MA/MB of the toe-crown-side thin part and heel-sole-side thin part within a range of 200 to 60%, the rebound performance was checked wherein a tendency similar to Table 1 was displaced.